In order to determine the existence and quantity of microbes or other analytes in foods and clinical samples it is usual to remove them from the sample and disperse them in water or other liquid. It is common to suspend the microbes from a 10 gram sample into 90 ml of liquid, a total volume of 100 ml. The suspension preferably contains a minimum of debris suspended from the sample, since it may interfere with the analysis. It should be noted that microbes attached to surfaces are relatively easily removed, whereas those situated internally in pores or folds or otherwise trapped in foods require a greater effort to disperse them. Many techniques and devices have been developed with the aim of obtaining representative removals of the microbes from test samples, for example, swabbing the surface or pressing agar contact plates on them. However, the variability of microbial adhesion makes swabs and contact plates very imprecise in their yield of microbes.
Techniques in which a portion of the test sample is excised and blended in liquid in a blender or other device which breaks up the sample and disperses the microbes generally produce microbial suspensions that reflects more accurately the microbial load of the sample. However, a blender is inconvenient because the suspensions it produces contain a great deal of debris, and because it must be cleaned and sterilized after use. An apparatus in which the test sample is sealed in a plastic bag and kneaded and crushed by electric-motor driven paddles to produce representative microbial suspensions is described in U.S. Pat. No. 3819158. The microbe-suspending action of this apparatus depends on the crushing action and on the motion of the liquid as it is driven from side to side in the bag. This apparatus is convenient because the sample-containing bag may be discarded after use, thus eliminating the need to clean and sterilize the apparatus. However, the crushing action on samples still produces suspended debris and prevents the apparatus being used for samples containing hard objects such as stones or shell because these pierce bags and cause them to leak. Also, such crushing devices cannot safely use transparent glass or plastic doors and it is thus not possible to see the state of the sample without switching off the apparatus and opening it up.
In experiments on microbe removal from foods I have shown that a crushing action is rarely necessary and that high rates of liquid shear alone are very effective at suspending microbes. For example, the microbe suspending action of devices known as "vortex stirrers" is quite good but these devices are inconvenient because they cannot be used with large volumes or with disposable plastic bags. Similarly, spraying samples with liquid by means of a spray gun for which the impacting of the liquid spray causes a strong shear is effective; however, it is difficult to collect the microbe-containing liquid when the test surface is at certain angles to the vertical.